1. Field of the Invention
The present invention relates to a light source and, more particularly, to a high-efficiency polarized light source device for a liquid crystal projector.
2. Description of Prior Art
In a conventional liquid crystal (hereinafter referred to as LC) projector, the LCd is play panel is normally rectangular, but the cross-section of the projecting beam emitted from the light source is circular. Therefore, the light energy distributed in the circumferential areas is sacrificed in order to irradiate the whole LC display panel. Moreover, since an LC projector requires polarized light, half the light energy is lost while the non-polarized light emitted from the light source is being polarized.
Because of the above problems, the brightness of the display in a conventional LC projector is not sufficient for image projection. One solution is to provide a light source with a higher power. However, this causes some other problems in that this approach not only consumes much more electricity, but also generates undesirable heat that will cause further problems.
In order to overcome such problems, other optical systems have been developed in the prior art. For example, and referring to FIG. 1, U.S. Pat. No. 5,098,184 discloses an illumination system for an image projection apparatus. The illumination system comprises a radiation source 22, a concave reflector 24 and a first and a second lens plate 26, 28 each being provided with a matrix of lenses for forming superimposed images of the radiation source on the object to be illuminated. The first lens plate 26 and the second lens plate 28 are utilized to redistribute the light intensity. Furthermore, the shape of each lens 27 and lens 29 corresponds to the shape of the LC display panel 20. Thus, this invention can provide a uniform brightness and efficiently make use of the light energy. However, that half the light energy is lost while converting the non-polarized light into polarized light still remains a problem.
In order to improve the efficiency of the LC projector, it is important to reduce the light energy lost while generating polarized light. A prior art entitled "Ultra-High-Efficiency LC Projector Using a Polarized Light Illuminating System" has been disclosed in SID 97 DIGEST, pp. 993 to 996, by Nakamura et al.
Referring to FIG. 2, the illuminating system includes a light source 30; a reflector 31; a first lens plate 35; a second lens plate 38; a polarizing beam-splitter array 140; a plurality of half wave plates 145; and a condenser lens 50. The first lens plate 35 includes a plurality of rectangular lenses 36 having a geometrical shape similar to the liquid crystal panel 5.
The second lens plate 38 includes a plurality of lenses 139 corresponding to the lenses 36 included in the first lens plate 35.
The polarizing beam-splitter array 140 includes a plurality of beam splitters, which is placed in the rear of the second lens plate 38 for splitting and polarizing the light beams into s-polarized light beams and p-polarized light beams.
The plurality of half wave plates 145 corresponding to the polarizing beam-splitter array 140 are placed on the paths of the s-polarized light beams or the paths of the p-polarized light beams to output alight beam having the same polarization. And the condenser lens 50 projects the light beam onto the liquid crystal panel 5.
In the illuminating system described above, the non-polarized light beam is converted into p-polarized light or s-polarized light by using a plurality of polarizing beam-splitters 140. Each polarizing beam-splitter can optionally pass the p-polarized light or the s-polarized light. The half wave plates 145 are alternately disposed at the output of the polarizing beam-splitter. Refer to FIG. 3, for example, while the non-polarized light beam P+S is incident into the polarizing beam splitter 141 through the lens 139, the p-polarized light beam P1 is transmitted through the polarizing beam splitter 141 and the s-polarized light beam S1 is reflected. The p-polarized light beam P1 is then passed through the half wave plate 145 and converted into an s-polarized light beam S2. Thus the light beam output from the polarizing beam splitter is s-polarized light beam S1+S2. In other words, the light energy of the light source device being inputted into the polarizing beam splitter is totally converted into a light beam having the same polarization. The performance of the LC projector can be markedly raised. However, the fabrication of the illuminating system is too complex. A plurality of tiny polarizing beam splitters have to be cemented together. It is very difficult to exactly align the surfaces coated with a semi-reflecting coating for each polarizing beam splitter to be parallel with each other. Furthermore, the position of the halfwave plate has to exactly correspond to the polarizing beam splitter. That is, only one of the light beams split by the polarizing beam splitter passes through the half wave plate, while the other one does not. Moreover, the alignment of the polarizing beam splitter in the LC projector must be precise. This causes some inconvenience to make use of such an illuminating system.